This invention relates to the control of particulate matter in flue gases, and, more particularly, to a control system for the addition of conditioning agents to the flue gas.
In a coal-fired power plant, coal is burned to heat air, which in turn boils water to form steam. The steam drives a turbine and thence an electric generator, producing electricity. Besides heat, the burning of the coal produces gaseous pollutants such as sulfur and nitrogen oxides, and a solid particulate known as fly ash. Environmental protection laws mandate that the amounts of gaseous pollutants and solid particulate emitted from the power plant be maintained at acceptably low levels, and the present invention deals generally with the technology for controlling particulate emissions.
One widely used approach for removing the particulate fly ash from combustion or flue gas streams is electrostatic precipitation. The flue gas stream with entrained particulate is passed between electrically energized electrodes that charge the particles so that they are attracted to, and deposited upon, a collection electrode. The particulate may optionally be charged prior to entry into the precipitator to increase the efficiency of removal. The cleaned combustion gases are released to the atmosphere, and the precipitated particulate is removed from the collection electrode.
The efficiency of operation of electrostatic precipitators depends markedly upon the electrical resistivity of the particulate. If the resistivity is too high, a collection current cannot be induced to flow between the electrodes of the precipitator, so that the ability to collect particulate is reduced, sometimes severely so. There exist conditioning procedures and apparatus for altering the conductivity of the particulate by injection of conditioning agents into the combustion gas stream prior to its entering the electrostatic precipitator.
An example of such a conditioning procedure is that disclosed in U.S. Pat. No. 3,993,429, and this approach has become widely accepted and used throughout the United States and the world. In this approach, a conditioning gas such as sulfur trioxide is injected into the combustion gas stream. The sulfur trioxide conditioning gas reacts with water vapor in the gas stream to produce sulfuric acid that is deposited upon the surface of the particulate. The sulfuric acid reduces the electrical resistance of the particulate, which is equivalent to raising the electrical conductivity of the fly ash particulate, so that the electrostatic precipitation treatment works well. Conditioning treatments are routinely used where the sulfur content of the coal burned in the power plant is so low that the electrical resistivity of the resulting particulate is too high to permit the electrostatic precipitators to operate with optimum collection rates.
The amount of conditioning agent added to the flue gas should be selected to optimize the precipitator performance. At the present time, the control procedures are performed manually, based upon the observations of the operator, or by a feedback control based upon maintaining a particular value of the power consumption of the electrostatic precipitator. Neither of these approaches works in a fully satisfactory manner, the former because of its lack of reproducibility and the latter because the functional performance of the electrostatic precipitator depends upon a variety of factors which are not reflected in a single power consumption figure. There exists a need for an improved approach to controlling the addition of conditioning agent to the stream of flue gas, so that the adjustments may be done automatically and reproducibly. The present invention fulfills this need, and further provides related advantages.